March 16, 2013 CFBC BOILER TUBE FAILURE DIAGNOSIS REPORT By K.K.

Parthiban, Venus Energy Audit System Agenda for visit The visit was made on 13th to 14th March 2013 to diagnose the various tube failures experienced in the CFBC boiler. The CFBC boiler was supplied by CVL about an year ago. The boiler rated parameters are 115 TPH, 108 kg/cm2a, 540 deg C with feed water temperature at 210 deg C. The boiler experienced failures at locations below: Radiant superheater bottom tube failure and secondary damage at nearby waterwall. Roof tube failure close to the radiant superheater outlet. Furnace wall tube failure close to kink date of failure 9th March 2013 Furnace wall tube failure close the panel to panel joint at 16 m level- 12th March 2013

The visit was made for diagnosing the failure. TUBE FAILURE DIAGNOSIS The tube samples were not available for the first two failures. However for the remaining two failures (9th March 2013 and 12th March 2013), the tube samples were available. Both the samples photographs are enclosed in annexure 3 & 4. The location of failures is marked in the drawings and enclosed in annexure 3 & 4. Thickness survey was done near the failed zone and at the kick off zone and are presented in annexure 5. First failure- radiant SH bottom tube failure- See annexure 1 The refractory choice for radiant superheater bottom should be suitable for rapid heat cycles and for high sulfur duty. Accmon 40 / Accmon 60 do not meet this requirement. Generally plastic refractories are applied for critical locations. In many plants, plastic refractory is considered when the high alumina castable with cement binder gives way. The failure cause is the refractory failure only. See annexure 1. Similar failures were experienced at another plant. See photo in annexure 1. In case the failure is repeated, the density of studs has to be increased so that the phoscast refractory can hold to the studs well. Second failure Roof tube failure- See annexure 2 The failure at roof is due to high air ingress through the leakage developed at sealbox. Such failures are seen at many plants. The radiant Superheaters are hung from roof panels. The radiant SH panels are hung close to the front wall. This is not an area of high dust concentration. It is seen that the sealbox design is not leak proof. Normally expansion bellows are provided between radiant superheater and the roof panel, as the radiant superheater expands upwards. See annexure 2, for a drawing showing the fabric bellows at another plant. Few other observations are noted on review of the drawings. These are highlighted

in the annexure 2. The radiant SH outlet sealbox height is only 75 mm as against 150 mm shown in another drawing. When the sealbox height is 150 mm at the inlet, it cant be lesser at the outlet. It is seen that coir rope is used around the radiant SH inside the sealbox. This will burn off in first firing itself, leaving the flue gas to fill inside the sealbox. The sealbox would warp and crack at the welds. It is advised to change the sealbox design and provide an expansion bellow as per our drawing enclosed with this report. The metallic bellow design is suggested, since the ID fan draft is already less in this boiler and positive sealing is a must. Third failure- furnace wall tube failure- See annexure 3 The boiler maker has provided refractory for furnace tubes at waterwall corners. This is not a feature for boilers of other makes. See photographs in annexure 3. If there had been any extra refractory, guiding the bed material towards the nearby tube, then this failure occurs. Typical failures are enclosed in the annexure 3. It is necessary to check that the refractory is as per drawing. As per drawing the studs holding the refractory should have been on the fins. But in the failed tube, it is seen that the studs are welded on to the tube. It is recommended to go for thickness mapping on a regular basis with a separate team at every shut down opportunity. This will help to prevent localized erosion failures and know if there is any permanent correction to be made or not. As of now, it is advised to trim any excess refractory by checking with a template gauge. See the drawings enclosed with this report. It is advised to check the entire length of refractory and correct this. Both corner refractory and the kick off zone refractory are to be checked with gauges. Fourth failure - furnace wall tube failure- See annexure 4 The location of failure was at the front wall. The location is marked in the drawing and enclosed in the annexure 4. The failure was at the field weld joint at 16 m level, and adjacent to corner refractory. The fin cutting made at field joints is not seal welded from inside the furnace. The incomplete weldment has allowed the localized erosion. The fin to fin weld has overlaps, which is also a reason for localized erosion. The tube to tube out of line can also cause erosion. This is to be checked using a steel scale. It is advised to carry out seal welding from inside and grind off the excess reinforcement. There are some plates welded along the corner refractory. Such a detail is not done at other plants. It is necessary to check the tubes adjacent to these plates. This is due to the reason that the distorted plates can direct the bed material towards the furnace tube. The cause for the failures can be due to excess refractory in the corner. The distorted plate could have caused the localized erosion. The studs are wrongly welded on the tube instead of on the fins. Thickness survey at the kick off zone indicated comparatively higher thickness as compared to tubes in this field joint area. It is advised to make platform all around and carryout the seal run from inside and grind off any excess weldment. This work would call for good planning of manpower, tools and tackles. MIG welding may take less time as compared to arc welding. Carbide tip cutter with flexible

shaft grinding machine may be required. It is advised to carry out a mock up trial and estimate the time taken for a meter length of weld. Based on this work study, the estimate for total work has to be arrived at. General guidance to reduce erosion rate of CFBC furnace CFBC boiler furnace wall tubes erode in a gross manner and in localized manner. The gross erosion will be due to presence of stones / high iron content / high bulk density of bed inventory. This can be seen up to a length of 2 meter above the lower furnace refractory. Localized erosion can occur at places at following locations. o Where refractory contour is projecting in way to divert the bed material towards the fin to tube zone. o Where any foreign material is left behind after the erection. This can divert the material locally to a tube. o Where any fin to fin joint is out of plane. o Where tube to tube is out of plane. o Where refractory at kink zone is excess than that of what is required. o Where radiant SH sealbox refractory is terminated. There may be erosion at the refractory edges. o Where another sealbox refractory is projecting in to the furnace.

It is advised to look for such spots regularly and take corrective action. Localized erosion can be accelerated by the high bulk density bed material / high iron bed material. The sky climber will be required for quick & detailed inspection of the furnace wall tubes. OTHER OBSERVATIONS Inadequate margin in ID fan During the visit it was informed that the ID fan margin was practically not available. On review it was found that for design purpose the fuels selected were on air dried basis. As fired moisture contents are not seen in the fuel specification. Indonesian coal moisture normally ranges from 28% to 40%, whereas the fuel moisture considered is 15.2%. This takes away little margin. The fan flow requirements were calculated and presented in annexure 6. For the case of Indonesian coal with 40 % moisture, the MCR gas flow for 20% EA condition will be 28.56 m3/s, whereas the ID fan is selected for 33.63 m3/s. There is margin available. But this is not available when the oxygen content increases to 9% due to air ingress or due to other operational reasons. When the air flow is adjusted for 9% O2 in flue gas, the excess air will be 75%. The MCR gas flow then becomes 42 m3/s as compared to available flow of 39.7 m3/s. Since the head at design point is 543 mmWC, the fan is able to cope up with present gas flow at 120 TPH steam generation. The air ingress may be too high disrupting the fan capacity. The air ingress at Radiant SH sealbox has to be arrested as it leads to secondary combustion and thus many parameters are affected. See annexure 6 on fan sizing for different fuels. At present the boiler seems to be operating on high excess air. This can be due to loss of fines from upper furnace and due to air ingress.

Air nozzle failures The design of air nozzle is not as per the current practice. The present air nozzle does not get adequate cooling air at the top. The air nozzle has four tier holes. Only one fourth of the air comes to top for cooling. Hence the top plate oxidizes and simultaneously the oxide scales get stripped too. After thinning the nozzles fail. It is suggested to increase the pitch between nozzles as per drawing no. The air nozzle shall be modified as per drawing no. The nozzles supplied for other customers by Cethar are similar to Chinese design. See the photographs in annexure 7. Duct size checking The duct sizing is checked for adequacy for the air required / flue gas produced in the case of Indonesian coal with 40% Excess air. The results are presented in annexure 7. Regular thickness mapping Tube thickness checking needs to be documented for future reference. Like bed coils of AFBC boiler, CFBC boiler would also erode. It is necessary to know the pattern of erosion in the boiler and take necessary action. The tube thickness meter shall be with a curvature probe suitable for tube OD of 38.1 mm 51 mm. The thickness range has to be 2 - 6.5 mm. The additional probe shall be procured. ESP outlet duct It is better to avoid a T duct when the gas is divided among two ID fans. Instead it should be made of 2 x 90 deg bends. This helps to save power in ID fan. See suggestion vide drawing. Effect of air ingress at roof Air ingress at roof panel can cause a localized secondary combustion and upset the gas temperature profile. Due to this secondary combustion the boiler exit temperature can be higher. Also the superheater spray can be more. At present nearly 80 deg C is being dropped at two desuperheater put together. Loss of fines from furnace can cause high combustion temperatures. Fines in upper furnace help for heat transfer to furnace wall tubes. High gas temperature at ESP inlet It is seen that the gas temperature is as high as 160 deg C as against 140 deg C. The primary APH is being bypassed to control the bed temperature. It may be necessary to fine tune the PA air flow & bed height to get the lowest bed temperature. Overloading the boiler can also have an effect. Once the air ingress at roof is arrested, there may be improvement in this. Lower feed water temperature Feed water temperature after HP heater is seen to be 201 deg C as against 210 deg C. This also leads to overloading the boiler. The turbine extraction conditions have to be checked with that of design. If the inlet steam temperature is less, then feed water temperature will be less. Also check the level control valve setting.

Higher moisture as compared to design At present 40 - 45% moisture coal is being fired as against the design moisture of 13.5 % in the Indonesian coal. The flue gas quantity is more due to the steam in the flue gas. The efficiency of the boiler is less with high moisture coal. This calls for burning additional coal and thus the flue gas flow is more. A 40% moisture coal will have a boiler efficiency of 83.2 % as compared to 28% moisture coal with a boiler efficiency of 86%. Bed material requirement It is learnt that considerable bed material is required in order to maintain iron content & bulk density under limits. Iron content should not cross 15% by weight. Bulk density can be 1350 kg/m3 at the maximum. Using high ash coal will solve the problem of high bed material consumption and the high bulk density. Recycling of drained bed material is permitted after the removal of iron by a magnetic separator. Usage of low ash coal with iron content in ash Indonesian coals have less ash. The iron content in ash can be as high as 20%. This gets concentrated in the furnace and increases the iron content. Higher ash content of nearly 10% helps to prevent iron accumulation in the bed. This is due to the reason that the bed ash generation will also be more and the draining will be more and hence concentration of iron is not seen. Procurement of right type of coal A simple criterion is the ash softening temperature. It has to be above 1200 deg C under reducing condition. This will automatically be low iron coal. While procurement of coal, the ash report must be scrutinized and the coal must be selected. Measurement of iron in fly ash Measurement of iron in fly ash helps to know whether the ash is harmful or not. If the fly ash iron content is in the increasing trend, it means the iron input is more. When using several coals, this number must be monitored. Iron can be tested by a permanent magnet. Lean bed height measurement The lean bed height can be measured close to kick off zone. This helps to know the lean bed height. The ash content in coal being fired may be so less, that it does not compensate for the loss of material from furnace. It is learnt that bed material is being used to a large extent in this boiler. The loss of fines from roof will also increase the bed material requirement. Condensate pH At present condensate pH is around 9.2. It may be necessary to increase the pH further by dosing morpholine or Cyclohexylamine at TG exhaust. This is due to the reason that the ACC tubes may undergo corrosion. It is advised to inspect ACC tubes from inside the ACC duct. See some good and bad ACC tubes. It is possible to check silt density index after CEP to know whether ACC tubes are

undergoing pH corrosion or not. In annexure 7, some photographs showing ACC corrosion are presented. Keeping water below the ACC structure The performance can be studied by measuring the tube temperatures with IR camera with and without water. There may be some benefit. The heat pick up by water vapor will contribute for maintaining lower air exit temperature. Inspection of steam drum / deaerator tower / deaerator storage tank / condensate tank / ACC tubes and ACC duct The above are to be inspected when there is a normal shut down without any forced cooling by cold water. From the color of the inside surface one can understand corrosion of metal. Magnetite layer is grey in color. See photo in annexure 7. Loop seal fan VFD and air flow measurement At present loop seal is operating at constant speed. There may be excess air flow at loop seal disturbing the seal and down leg. The rotometers provided at loop seal air pipe are to be lowered for visibility. The loop seal blower has to be operated with a VFD to regulate the air required for maintaining the seal and the bed material transfer. If VFD is not planned at least there has to be a dump valve to send part of the air to PA duct. Difference between the DCS oxygen reading and the portable meter reading The data on O2 readings between DCS and Field were seen. The field readings are higher by 5%. There can be problem in the sensor or there can be air ingress at measurement points. ID fan discharge damper removal The ID fan discharge dampers are only pressure killers in case it is not used for maintenance. There may be some gain in head, if the dampers are removed. Fly ash sieve analysis ESP first field ash must be analyzed for size. It is advised to procure sieve shaker and sieves with 25, 50,75 and 100 microns mesh openings. On regular analysis, one can come to know of fines loss from furnace. If fines are lost, there can be loop seal plugging / vortex finder distortion. Explosion doors in the furnace Explosion doors are advised to avoid furnace damage during any furnace explosion. Furnace explosion can be sensed by using CO analyser. Fuel accumulations in the fuel chute can cause explosive situations. Explosion door provision will call for pressure part modifications.

Photo 1: It is learnt that the radiant SH bottom refractory got dislodged in the first few months of operation. This has led to erosion of the radiant SH. There had been secondary damage to furnace walls. Improper selection of refractory and improper casting method and inadequate anchor density can cause this problem. Phosphate bonded high alumina castable is the right choice. The stud density shall be as per AFBC boiler bed coil if the refractory failure is seen again.

Photo 2: There was no photo available for the radiant SH failure. This is a typical failure at wingwall SH at another plant due to improper anchors and wrong selection of refractory.

ANNEXURE 2: PHOTOS AND COMMENTS RELATED TO SECOND FAILURE AT ROOF PANEL

Photo 1: The second failure was reported to have taken place at the roof panel close to the radiant SH sealbox. This is due to break down of sealbox and the resultant air ingress.

Photo 2: These are the additional views of the sealbox. The sealbox height should have been 150 mm as per another drawing. The coir rope would burn off in no time and the hot gases would be filled inside the sealbox. This leads to high heat input. The sealbox will naturally fail due to this. For this reason there had been flue gas leakages. Ideally the sealbox should be fully filled with castable. Already there is a sleeve to protect the SH coils. That is the reason that the roof tubes failed instead of SH tubes. Modifications are suggested in the sealbox. An expansion bellow shall be used to ensure positive sealing. This is required as the SH coils expand upwards due thermal expansion. A drawing is given.

Photo 3 & 4: Two different details are seen between Cethar drawings. The sealbox height is 150 mm. There is no coir rope detail in this. Incidentally this is the better detailing as compared to coil rope case. It is advised to increase the sealbox height to 150 mm and pour fresh castable refractory. It is advised to install the metallic expansion bellow as per enclosed drawing. The space should be filled with ceramic blanketing.

Photo 5: The above drawing is taken from the Chinese boiler. They have provided fabric expansion bellow for positive sealing of the sealbox. Fabric joints stand a chance of bursting on furnace back pressure. Hence fabric joint is not suggested here.

Photo 1: The third failure was at kick off zone. This boiler is provided with corner refractory from top to bottom. The failed tube was close to the corner refractory.

Photo 2: Detail K shows the typical corner refractory detail which is provided from top to bottom of the furnace walls. If the refractory contouring is improper, there can be localised diversion of the bed material towards to the fin to tube area. This results in localised erosion. There are no anchors on the tubes. The anchors are only on the fins.

Photo 3: The above is the failed tube available at plant. The erosion is seen to have originated at the tube to fin joint zone. The studs are seen welded on the tube instead of on fins. With this, the refractory contouring would not be as per engineering requirement. If there was any excess refractory, then the erosion will be localised.

Photo 4: It was seen that two tubes are removed from the failed area. This extended refractory will again cause erosion. There should be no hooks. Any excess weldment at tube to tube joint, fin to fin joint and fin to tube joint are to be removed. The purpose of providing kick off zone is lost.

Photo 5: The above is the detail of refractory at kick off zone given by boiler maker for this project. This is likely to cause erosion. The detail given for some other projects by the same boiler maker is as below.

Photo 6: The above are the details of kick off zone refractory in some other projects by the same boiler maker. This detail will ensure less erosion rates at kick off zone.

Photo 7: This photo shows the high erosion rate when the refractory is excess. This is from a Chinese boiler.

Photo 8: The above are some failures in CFBC furnace wall failures, as the erection debris is left behind by the erector. The material gets diverted to localised spots and cause failure.

Photo 1: The location of the 4th failure is marked in the drawing. The failure was close the field joint area and next to the corner refractory. See further photos below.

Photo 2: The photo shows the tubes after removal. The first tube close to refractory had burst. There is plate welded nearby. This location is just below the field joint (panel to panel joint).

Photo 3: The photo shows the tube that burst open near the corner refractory.

Photo 4: It can be seen that the fin to fin joint is not as per recommendations. The fin weld should have been done from furnace side.

Photo 5: The third tube from the failed tube also thinned down. During thickness measurement this was identified.

Photo 6: Close inspection of the fin joints revealed that the fin welding is not from inside. There is overlap of fin. The welds are not dressed. All the above requirements are seen in the drawings. The water marks show the pattern of material flow during normal operation.

Photo 7: These drawings are taken from the pressure part drawings of the boiler supplier. We can see that the fin joints can have a negative offset of 1.6 mm at the maximum. Positive offset is not allowed. Similarly tubes to tube joint can be with 1 mm offset. The bottom tube can go inside but not outside. The fin to fin joint is to be made from combustor side.

Photo 8: The third tube which showed sign of erosion near the fin. The fin to fin weld was made from outside only. This is the area where the fins are slit at shop for field matching of the tube to tube butt weld joint.

Photo 9: The erection note says that the inside welds between panels are to be flush ground smooth. Any projection leads to drifting of the bed material towards the fin base causing thinning down of tube.

Photo 10: It is seen that SS plates are welded at the fins along the corner refractory. This is not continuous. It is also seen in warped condition since it is not cooled well. It can cause localised erosion of the tubes since it is warped. These SS plates are not provided in other boilers of Cethar.

Photo 11: As per the drawing of Cethar, the SS plate is to be welded to the tube. The erection is not as per drawing. However this is not a good idea. Only refractory must be shaped to the required profile. It is advised to make a gauge and check entire corner refractory. See drawing attached with the report.

Photo 12: This is a photo taken in the furnace elsewhere at the panel to panel joint. The fin to fin weld is not done and not dressed too. The adjacent corner refractory is seen haphazard. Even the tubes are seen to be out of plane.

Photo 13: As per the drawing the fin to fin weld at field joint area should have been seal welded from inside. It is seen that the fin to fin welding is not proper and the seal welding was done from outside.

Photo 14: The photo shows the slits made at factory for the purpose of tube matching at field joint. The weld has to be full penetration weld. The second side is also to be seal welded.

Design by: Name: Sign:

Approved by: Name: Sign:

ANNEXURE 7: PHOTOS AND COMMENTS RELATED TO OTHER OBSERVATIONS

Photo 1: The air nozzles are seen to get oxidised and eroded. It is learnt that at every boiler stoppage there are some worn out nozzles. This design is already changed by the boiler supplier, based on our feedback at Jaibalaji, Durgapur.

Photo 2: The photo shows the design of air nozzles used in CFBC boiler in India. The nozzles are somewhat similar in all cases. The air inlet pipe is close to the air nozzle head. The air hits the top and travels down. This keeps the head cooled better.

Photo 3: There is the current air nozzle design adopted by Cethar. .

Photo 4: This is another air nozzle design by BHEL. The air is released from bottom. The head is cooled better.

Photo 5: The fan sizing is compared here. We can see for the current operating condition, there can be limitation in volume flow rate of ID fan.

Photo 6: The T duct at ESP outlet is not a good feature. This results in turbulence at this place. If the duct is modified as per the drawing given, there will be improvement over draft loss.

Photo 7: This is the check on duct sizing for Indonesian coal.

Photo 8: ACC corrosion due to low pH. Like this ACC tube inside has to be visually checked.

Photo 9: This is a curve produced by EPRI scientists after extensive tests with ACC corrosion. Before raising pH, SDI must be tested. After raising the pH SDI must be tested. The pH can be boosted by dosing morpholine at TG exhaust. Dissolved iron does not reveal this corrosion much.

Photo 10 & 11: The corrosion products of steel can be seen in the steam drum of the photo at the bottom. The top photo shows the steam drum with proper feedwater and boiler water chemistry. The grey color layer is magnetite layer.